Conventional techniques for homogenizing light make use of arrayed micro-lenses, diffractive diffusers, ground glass diffusers, and holographically-generated diffusers. Micro-lens arrays homogenize light by creating an array of overlapping diverging cones of light. Each cone originates from a respective micro-lens and diverges beyond the focal spot of the lens. In the conventional arrays, the individual lenses are identical to each other. Ground glass diffusers are formed by grinding glass with an abrasive material to generate a light-scattering structure in the glass surface.
Micro-lens arrays, ground glass diffusers and holographic diffusers all have the disadvantage of not being able to control the angular spread of the homogenized, diverging light. Light in general has an angular spread that is fairly uniform over a desired angular region, but the boundaries of the angular region are blurred. With the conventional diffuser methods, the energy roll-off at the edge of the desired angular spread can extend well beyond this region.
Diffractive diffusers can be used to control the angular spread of the output light, but such diffusers are limited with respect to the amount of spread that they can impart to the output light. Due to fabrication limitations for short wavelength sources, visible or below, and limitations in the physics of the structures for longer wavelengths the maximum angular spread is limited. Further, diffractive diffusers used in their traditional binary form can include a significant amount of background energy and the patterns must be symmetric about the optical axis.
To overcome said disadvantages of these conventional devices, US20070223095 discloses an optical device having a plurality of square facets formed by a plurality of optical elements. The facets are used to direct portions of an incident light beam in predetermined, respective directions. The facets are formed adjacent to each other in a two-dimensional array. The locations of the facets in the array are random with respect to the directions of the corresponding light beam portions. It is a disadvantage of the known lighting device that it has a relatively poor performance and/or is relatively large.